The capability of modulating the intensity of light in an optical fiber at high frequencies is essential to the development of very high speed optical communications, advanced sensors, and high frequency signal processing. A variety of techniques have been developed, the most important being gain modulation of a semiconductor diode laser and external modulation of a continuous wave source in an integrated optic modulator. The most successful devices for the latter approach are based on waveguide Mach-Zehnder interferometers implemented in lithium niobate (LiNbO.sub.3). The Mach-Zehnder interferometer is an optical device wherein input light is split and travels along two continuous paths, and is recombined. The two optical paths may be of different lengths so that on recombination the two beams may interfere either constructively or destructively. Lithium niobate is an electro-optic material, such that its index of refraction, and thus the optical path length traveled by light passing through the material may be varied by the application of an electric field. In a lithium niobate-Mach-Zehnder amplitude modulator, an electric field (which may be modulated) is applied across one of the interferometer arms (or an opposite field is applied across the two arms) to vary the interference at the output. For very high frequency operation, a "traveling wave" geometry is used wherein the applied electric field propagates down the electrodes, which are constructed as a microwave waveguide, at the same speed as the light propagates through the optical waveguide. The modulation on the input electrical signal is thus transferred to the intensity of the output light.
Despite the broad application of Mach-Zehnder amplitude modulators in both analog and digital applications, they have a number of drawbacks. Chief among these is the problem of bias drift. In operation, the modulator is typically required to be operated about a particular point in its transmission characteristic, i.e. at a particular bias. For example, for linear operation as required in analog systems (e.g., cable television distribution or radar applications), the Mach-Zehnder modulator is operated at the 90.degree. (quadrature) bias point. Improper bias causes undesirable effects in the transmitted optical signal, such as increased harmonic distortion in analog systems and inter-symbol interference in digital systems. In general, it is impossible to fabricate a modulator with the proper intrinsic bias. Thus the bias is usually set by applying DC voltage. Furthermore, the required bias voltage is not absolutely fixed: It may vary with time due to external environmental factors (e.g. temperature, acoustic effects) or internal factors (intrinsic field screening by long-term charge transport in lithium niobate or silicon dioxide layers). The latter source of bias drift is particularly pernicious, as it can easily swing the bias phase over a full 360.degree. (2.pi. radians) on time scales of the order of several months, so that some means of complicated feedback controlled bias tracking is essential to extend the usable lifetime of a modulator in practical application. This problem has also spurred considerable research, mostly unsuccessful, toward the development of an intrinsically more stable device. A second drawback is the relatively high insertion loss of Mach-Zehnder devices (3-5 dB, typically). This loss results from the bends in the optical waveguides in the Mach-Zehnder configuration. Improving both of these features is the subject of a number of research efforts worldwide.
Clearly, then, an amplitude modulator with an intrinsically stable bias would have great impact in communications, electronic warfare, and other applications. A reduction in the intrinsic loss would be beneficial as well.